1. Field
The present disclosure relates generally to electronics, and more specifically to transmitters and receivers.
2. Background
In a radio frequency (RF) transceiver, a communication signal is typically received and downconverted by receive circuitry, sometimes referred to as a receive chain. A receive chain typically includes a receive filter, a low noise amplifier (LNA), a mixer, a local oscillator (LO), a voltage controlled oscillator (VCO), a baseband filter, and other components, to recover the information contained in the communication signal. The transceiver also includes circuitry that enables the transmission of a communication signal to a receiver in another transceiver. The transceiver may be able to operate over multiple frequency ranges, typically referred to a frequency bands. Moreover, a single transceiver may be configured to operate using multiple carrier signals that may occur in the same frequency band, but that may not overlap in actual frequency, an arrangement referred to as non-contiguous carriers.
In some instances, it is desirable to have a single transmitter or receiver that is configured to operate using multiple transmit frequencies and/or multiple receive frequencies. For a receiver to be able to simultaneously receive two or more receive signals, the concurrent operation of two or more receive paths is generally required. Such systems are sometimes referred to as “carrier-aggregation” systems. The term “carrier-aggregation” may refer to systems that include inter-band carrier aggregation and intra-band carrier aggregation. Intra-band carrier aggregation refers to the processing of two separate carrier signals that occur in the same communication band. Inter-band carrier aggregation refers to the processing of two separate carrier signals that occur in different communication bands.
One of the challenges in receiving multiple signals in a receiver is preventing out of band (OOB) signals, referred to as “jammers” or “blockers” from interfering with the desired receive signals. Examples of OOB blockers include, for example, transmit energy from a nearby transmitter that can interfere with the receive signal due to the proximity of the transmit antenna to one or more receive antennas, and other blockers, such as wireless fidelity (WiFi) signals. One way of preventing OOB blockers from interfering with the desired receive signals is to implement a surface acoustic wave (SAW) filter. A SAW filter provides effective blocker rejection and a low insertion loss, but generally must be fabricated as a discrete component, and is not tunable. Accordingly, in a multi-band receiver the front-end is typically implemented using a number of band-select switches to direct the receive signal to the appropriate SAW filter. The SAW filter is then typically connected to a dedicated low noise amplifier (LNA). Typically, a matching network is also required between the SAW filter and the LNA to obtain good impedance matching.
Existing tunable filters sometimes use tunable or adjustable capacitances, sometimes implemented using a digital variable capacitance (DVC). Unfortunately, adjusting filter characteristics using an adjustable capacitance tends to lower the quality factor (Q factor) of the filter.
Therefore, it would be desirable to be able to process multiple receive signals in multiple receive bands using a tunable filter that can be reconfigured to the desired band and that maintains a high Q factor, thus eliminating the need for a SAW filter and matching network for each receive band.